This invention relates to novel in-pile parts for a nuclear reactor made of nickel base alloy and method of heat treatment therefor, and more particularly it relates to in-pile parts for a nuclear reactor made of nickel base alloy which are free from stress corrosion cracking that might take place in pure water of high temperature and high pressure of a light water nuclear reactor and to a method of heat treatment for such parts.
Nickel base alloys are used for in-pile structures of a light water nuclear reactor. Of all the nickel base alloys used for this purpose, Inconel X750 nickel base alloy for the precipitation hardening type, i.e. Aerospace Material Specification (AMS) 5667H, has particular utility as material of high resilience for forming in-pile parts of various types because of its high heat resistance and high strength. This alloy consists of by weight less than 0.08% C, 14-17% Cr, 2.25-2.75% Ti, 0.7-1.2% Nb+Ta, 0.4-1.0% Al, less than 0.5% Si, less than 1% Mn, 5-9% Fe and the balance Ni. The in-pile parts as mounted in a nuclear reactor form a crevice between the parts and are subjected to high stress and exposed to pure water of high temperature and high pressure at all times. Thus, there are the risks that the in-pile parts would be corroded by the pure water and develop stress corrosion cracking due to the existence of crevices and the stress applied thereto.
The in-pile parts that tend to develop such crevice stress corrosion cracking include a finger spring 3 interposed between the tie plate 1 and the channel box 2 in a fuel assembly shown in FIG. 2, an expansion spring 6 for holding a graphite seal 4 in place within an index tube 5 in a control rod drive mechanism shown in FIG. 3 and a hold down beam 9 interposed between arms 8 for pushing downwardly an elbow tube 7 of a jet pump shown in FIG. 4.
A nickel base alloy heretofore used for forming such in-pile parts has been subjected to solution heat treatment, then subjected to aging treatment at a relatively high temperature (approximately 860.degree. C.) and thereafter subjected to aging treatment again at a lower temperature. Experiments conducted by the present inventors have revealed that the nickel base alloy treated in this way is not necessarily high in stress corrosion cracking resistance. In view of the results of the experiments, the present inventors have conducted research that has led to the present invention.
Further, the nickel base alloys used in a condition exposed to pure water of high temperature and high pressure in a light water nuclear reactor are known from U.S. Ser. No. 733,520 (May 31, 1967) and U.S. Pat. No. 3,574,604. However, the former alloy is not preferable because its Cr content is high so that the austenite matrix is unstable and it is liable to form precipitates which are harmful for resistance to stress corrosion cracking at high temperature. With respect to the latter alloy it is confirmed by the present inventors that since Nb content is extremely higher than Ti content the growth of precipitates phase is liable to occur when used at high temperature thereby degrading the stress corrosion cracking resistance.